1. Field of the Invention
The present invention relates to a semiconductor device. More particularly, the present invention relates to a semiconductor device applied to display devices such as a liquid crystal display device and organic EL (Electro Luminescence) display device.
2. Description of the Background Art
A thin film transistor is used in a display device. As an example of such a thin film transistor, an n type thin film transistor of an LDD (Lightly Doped Drain) structure disclosed in Japanese Patent Laying-Open No. 2001-345448 will be described hereinafter.
An n type thin film transistor of an LDD structure has a source region, a drain region, a channel region, an LDD region, a gate insulation film, a gate electrode, and the like formed on a glass substrate. In the n type thin film transistor, a voltage higher than that applied to the gate is applied to the drain to produce a relatively large electric field at the junction region of the drain side.
Electrons accelerated by the electric field induce impact ionization, whereby a pair of an electron and hole is generated. Impact ionization is repeated to induce more pairs of electrons and holes, whereby the drain current is increased to result in avalanche breakdown. The drain voltage thereof becomes the source-drain breakdown voltage.
In a thin film transistor of the LDD structure, an LDD region is formed at a region between the channel region and the source region, and at a region between the channel region and the drain region. The impurity concentration of the LDD region is set higher than that of the channel region and set lower than that of the source region and drain region. By alleviating the electric field in the proximity of the drain region through the LDD region, impact ionization is suppressed, whereby the source-drain breakdown voltage can be improved.
A thin film transistor of an LDD structure had the problem that the ON current of the thin film transistor is low since the resistance of the LDD region functions as parasitic resistance.
To solve this problem, a thin film transistor of a GOLD (Gate Overlapped Lightly Doped Drain) structure is proposed in Japanese Patent Laying-Open No. 2002-076351. An n type thin film transistor of a GOLD structure has a source region, a drain region, a channel region, a GOLD region, a gate insulation film, a gate electrode, and the like formed on a glass substrate.
The GOLD region is formed at a region between the channel region and the source region, and a region between the channel region and the drain region, particularly at a region located right under the gate electrode. The GOLD region is formed overlapping with the gate electrode in plane. Since the GOLD region is located right under the gate with a relatively low impurity concentration, a relatively high ON current can be obtained. Furthermore, relatively favorable source-drain breakdown voltage can be ensured.
However, the resistance of the GOLD region in a thin film transistor of a GOLD structure similarly becomes a factor of parasitic resistance.